Spotted knapweed (Centaurea biebersteinii DC.) is a perennial, invasive forb that infests millions of hectares of private and public rangelands in western North America. Previous research indicates that domestic sheep (Ovis aries) readily graze spotted knapweed, but landscape-scale prescriptive grazing of spotted knapweed has not been studied. We quantified the diets and forage utilization of a ewe–lamb band (about 800 ewes and 1 120 lambs) that prescriptively grazed spotted knapweed–infested foothill rangeland in western Montana in the summers of 2003 and 2004. In mid-June or mid-July, sheep grazed light and moderate infestations of spotted knapweed (13% and 36% of vegetative composition, respectively). Nutritive quality of sheep diets was similar to sheep grazing uninfested rangeland, and sheep exhibited few forage preferences or avoidances. Sheep diets averaged 64% spotted knapweed in the moderate infestation and 26% in the light infestation. Sheep in the light infestation ate fewer graminoids in June than July (17% vs. 55% of their diet, respectively; P  =  0.04), whereas sheep in the moderate infestation ate fewer graminoids in July (45% in June vs. 20% in July; P  =  0.09). In the moderate infestation, relative utilization of spotted knapweed was greater in July than June (50% vs. 35%, respectively; P  =  0.04), but averaged 46% in the light infestation. Previous research suggests that these levels of relative utilization may make herbicide application uneconomical. Relative utilization of graminoids was light in both infestations (15% in June or 31% in July). Our results indicate that sheep can prescriptively graze light or moderate spotted knapweed infestations in either June or July. Sheep consumption and relative utilization of graminoids will be less if light infestations are grazed in June rather than July. In moderate infestations, sheep will eat fewer graminoids and utilize spotted knapweed more heavily when grazed in July rather than June.